1. Field of the Invention
The invention relates to a bi-standard television receiver for signals modulated in frequency in accordance with a first standard in which a chrominance subcarrier is multiplexed in frequency (for example, PAL, NTSC), or in accordance with a second standard in which the signals are temporally multiplexed (for example, MAC), provided with a demodulator arrangement comprising, upstream of said frequency demodulator, a bandpass filter having a tunable central frequency which is controlled at the frequency of the input signal by means of a feedback returning the demodulated signal from the demodulator to the tuning control input of said filter, this being realized in the above-mentioned first standard via a first loop comprising a low-pass filter in parallel with a second loop comprising a bandpass filter substantially tuned to the frequency of the chrominance subcarrier, an adder for adding the signals from each one of the two loops and for applying them to the tuning control input of the filter having a tunable central frequency.
The invention is particularly used in satellite television for demodulating a signal at an intermediate frequency in the 400-700 MHz frequency range in which signals of the PAL standard as well as of the MAC standard may occur.
It is advisable to avoid the appearance of parasites referred to as "clicks" in the demodulated FM signal. It is known that such parasites occur in the presence of noise because the latter is powerful enough to cause a phase jump of more than 180.degree. of the input signal of the demodulator. The demodulated signal is perturbed to a greater extent as the useful signal at the input is weaker. The threshold is the ratio between the carrier and the noise at the input, measured in a band of reference noise for which the clicks disappear. There is a static threshold (i.e. measured with a continuous modulating signal: white picture) and a dynamic threshold (i.e. measured with a variable modulating signal: for example, test picture of colour bars): the latter is more difficult to lower than the static threshold. Dynamic thresholds are always meant where the threshold values are mentioned in the following description.
2. Description of the Related Art
To avoid the clicks, it is known to reduce the passband before the demodulator by means of a filter or to use a demodulator whose characteristics are chosen in such a way that it cannot follow the phase jumps, or to insert a filter which is controlled at the frequency of the signal.
A demodulator using the latter method is known from the document WO-A-86/03919. According to this document two different circuit diagrams (FIGS. 3 and 4) should be used, dependent on whether the reception is realized in accordance with the NTSC standard or in accordance with the MAC standard. Inter alia, a line delay element should be used to retrieve the phase information.